Any device which converts heat energy into mechanical energy is called heat engine. Heat engines of two types. External combustion engine(steam engine) and internal combustion engine (petrol or Diesel).

Petrol Engine vs Diesel Engine
Petrol Engine
Diesel Engine
Petrol engine uses petrol as fuel Diesel oil is the fuel in diesel engine
It is light compact machine and is most suitable for use in aeroplanes. It is compact, heavy and robust machine and unfit for use in aeroplanes because of heaviness.
A spark plug is required for initial ignition No spark plug is required to start ignition
The spark plug fails to produce the spark sometimes and breakdowns are quaint frequent. It has no spark plug and hence has fewer chances of breakdown.
The running cost is high as it uses petrol which is a refined oil and is fairly costly. The running cost is low as in it crude oil is used which is much cheaper than petrol.
The efficiency is low varying from 30-40%. The efficiency is as high as 40 to 55%
It is lighter and suitable for light vehicles like bike, auto etc. It is heavier and not suitable for lighter vehicles but suitable for heavy vehicles like bus, lorry etc.
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